The invention relates to a data carrier comprising the means specified hereinafter, namely receiving means adapted to receive and to supply an amplitude-modulated carrier signal which has been amplitude-modulated in dependence on data to be transmitted, and rectifier means to which the received amplitude-modulated carrier signal can be applied in order to be rectified and which are adapted to generate a d.c. supply voltage corresponding to the received amplitude-modulated carrier signal, and voltage limiting means adapted to limit the d.c. supply voltage generated with the aid of the rectifier means to a first limit value, and amplitude demodulation means to which the received amplitude-modulated carrier signal can be applied in order to be demodulated and which are adapted to supply a data signal representative of the data to be transmitted, and data processing means adapted to process the data signal supplied by the amplitude demodulation means, the data processing means having a power supply input, which power supply input is arranged to receive the d.c. supply voltage.
The invention further relates to a circuit comprising the means specified hereinafter, namely rectifier means to which a received amplitude-modulated carrier signal which has been amplitude-modulated in dependence on data to be transmitted can be applied in order to rectify the carrier signal and which are adapted to generate a d.c. supply voltage corresponding to the received amplitude-modulated carrier signal, and voltage limiting means adapted to limit the d.c. supply voltage generated with the aid of the rectifier means to a first limit value, and amplitude demodulation means to which the received amplitude-modulated carrier signal can be applied in order to be demodulated and which are adapted to supply a data signal representative of the data to be transmitted, and data processing means adapted to process the data signal supplied by the amplitude demodulation means, the data processing means having a power supply input, which power supply input is arranged to receive the d.c. supply voltage.
Such a data carrier of the type defined in the first paragraph and such a circuit of the type defined in the second paragraph are known, for example from the document WO 96/38814 A2. Such a data carrier serves for the contactless communication with a transmitting/receiving station. In order to transmit data from the transmitting/receiving station to such a data carrier (transponder) a carrier signal is amplitude-modulated in the transmitting/receiving station, which amplitude modulation is effected in dependence on the data to be transmitted.
In such data carrier systems comprising at least one transmitting/receiving station and at least one data carrier it is known to carry out an amplitude modulation of the carrier signal with a modulation percentage of 100%. The advantage of such an amplitude modulation essentially resides in the fact that the amplitude modulation means required for this can simply be implemented as an integrated circuit. A disadvantage of such an amplitude modulation with a modulation percentage of 100% is that the sideband signals which occur when this amplitude modulation is used have a comparatively high level, which often gives rise to problems in complying with national regulations as regards spurious emission.
In order to avoid the afore-mentioned problems as a result of sideband signals of comparatively high levels it is also known to use an amplitude modulation with a comparatively low modulation percentage, for example a modulation percentage of approximately 10%.
When such an amplitude modulation with a low modulation percentage is used a data carrier of the type defined in the first paragraph unfortunately has the problem that the voltage limiting means, which are connected to the rectifier means and which serve for limiting the d.c. supply voltage, derived from the received amplitude-modulated carrier signal with the aid of the rectifier means, to a first limit value, counteract the amplitude modulation in the amplitude-modulated carrier signal received by the receiving means and supplied by the receiving means, which results in the amplitude modulation being evened out. Unfortunately, during the demodulation of the received amplitude-modulated carrier signal by means the amplitude demodulation means this evening-out of the amplitude modulation, which is equivalent to a reduction of the modulation percentage, may lead to problems, as a result of which errors may occur in the data signal representing the data to be transmitted and supplied by the amplitude demodulation means, which is obviously undesirable.